The invention pertains to a process for the production of preforms for quartz glass optical waveguides, which is applicable to the production of optical transmission devices for communications technology. More particularly, the invention relates a method for purging a porous quartz glass tube formed in a hydrolysis process.
In optical communications technology, widespread use is made of optical waveguides based on quartz glass. These waveguides are drawn from quartz glass rods with a length of 1 m to 2 m and with a diameter between 2 cm and 10 cm, which are referred to as "preforms". One of the essential requirements of the preform material is that it must be of extremely high purity, because a concentration of impurities even in the ppb range (10.sup.-9) can cause severe absorption losses in the waveguide.
Several processes are known for the production of waveguides. These are described, for example, in the book "Optische Telekommunikationssysteme". The four most important processes in order of their economic importance are:
OVD (outside vapor deposition),
MCVD (modified chemical vapor deposition),
VAD (vapor axial deposition), and
PCVD (plasma-activated chemical vapor deposition).
In the MCVD and PCVD processes, a substrate tube of quartz glass is required, in which the waveguide material is then deposited. Although the same purity requirements are not imposed on the material of the substrate tube, it is nevertheless necessary to ensure very low OH concentrations, which means that the process used to produce the substrate tubes is correspondingly expensive. The same is true for the so-called "casing" tubes.
Because most of the light-related performance is concentrated in the core of the waveguide and in a thin, adjacent jacket zone, for economic reasons it is becoming increasingly common to produce only this interior region of highly pure waveguide material and to fuse a thick-walled tube (casing tube) over it, which consists of quartz glass with a somewhat higher concentration of impurities. Nevertheless, the OH content of a casing tube of this type must still be very low.
The OVD process (Corning/USA) is the most economically important process for the production of preforms and is used to produce most preform material. The process is characterized by the simplicity of the production process and by the possibility of producing very large preforms.
The process steps for the production of preforms by the OVD process are shown schematically in FIGS. 1 and 2. As FIG. 1 shows, first a large quartz glass body 1 of fine-pored material is built up in stages. For this purpose, a burner 2, which is operated with a mixture of methane (CH.sub.4) and oxygen to which silicon tetrachloride (SiCl.sub.4) is also added, is moved back forth along a rotating ceramic rod 3 (Al.sub.2 O.sub.3).
A chemical reaction in the flame, which is referred to as hydrolysis, leads to the formation of quartz glass. This glass is deposited in the form of layers of a white, porous material. This material, also called "glass soot" has under the control of the hydrolysis reaction, a density ranging from 10% to as much as 25% of solid quartz glass and consists of tiny particles of small diameter, which are fused to each other at individual points.
To produce doped quartz glass, germanium tetrachloride (GeCl.sub.4), for example, is added to the burner gases. As a result, a GeO.sub.2 -doped material with an increased index of refraction is produced.
During hydrolysis, both quartz glass and a large amount of water vapor are produced. High concentrations (about 1,000 ppm) of this water vapor are incorporated in the form of OH ions into the porous quartz glass.
After the desired amount of quartz glass has been deposited, the burner is turned off. Because of its higher coefficient of expansion, the substrate rod contracts during cooling to a greater extent than the quartz glass body, with the result that the rod can be pulled out from the quartz. Because of the high OH content of the material produced in this way is, it is still unsuitable for the production of waveguides and must be dried over the course of further process steps.
As illustrated in FIG. 2, porous quartz glass body 1 is placed in a furnace 5. In a first step, i.e., physical drying, the OH content in the quartz glass is reduced from about 1,000 ppm to about 10 ppm over the course of about 2 hours by purging it at 800.degree. C. with dry, ultrapure gases 6 (nitrogen, oxygen, helium).
In the second step, i.e., chemical drying, purging is carried out with chlorine or thionyl chloride gas (SOCl.sub.2) and helium. This chemically active gas mixture flows around the porous quartz glass body for several hours at about 800.degree. C.
The OH ions in the quartz glass beads diffuse to the surface and react there with the dry gas to form HCl gas, which is gradually transported out of the quartz glass body. As a result, the OH content in the material can be lowered to values below 0.01 ppm. This low OH content is required to reduce the OH absorption band in the waveguide pulled from the quartz glass to a value significantly below 1 dB/km.
Then the furnace temperature is increased to such a point that the porous glass body melts to form a solid rod.
In the production of substrate rods for MCVD and PCVD preforms and in the production of casing tubes, the porous quartz glass body is produced with a much larger inside diameter and is fused to form a solid tube after drying.
Because the drying process lasts many hours and calls for the use of aggressive drying gases, which must always be fresh and of high purity as they flow through the furnace, it represents a time-consuming and cost-intensive process step. The need to dispose of large amounts of waste gas also leads to considerable cost.